1. Field of the Invention
The present invention relates to scanning antennas and more particularly to antennas capable of circularly scanning a high directivity low sidelobe beam.
2. Description of the Prior Art
Many radar systems require radiation beams with broad elevation beamwidths and narrow azimuth beamwidths which are azimuthally scannable over 360 degrees. These radiation characteristics have been achieved with various antenna designs, one of which is the mechanical rotation of an entire antenna assembly comprising a large parabolic cylinder reflector and a feed system therefor. These are high inertial systems, however, requiring considerable driving power and providing maximum scan rates in the order of 25 rpm. An antenna capable of providing a fan beam rapidly scannable in the azimuthal plane, is disclosed in U.S. patent application Ser. No. 918,182, filed by Cronson et al on Jun. 22, 1978 and assigned to the Sperry Corporation. The antenna disclosed by Cronson et al includes a stationary transreflector, which may be an annulus of a spherical or parabolic torus, the surface of which is constructed of reflecting metal rods that are oriented at 45 degrees with respect to the meridians of the torus and a feed system that illuminates successive sections of the annulus as it rotates about the focal circle thereof. This rotating feed system produces an illumination pattern that is shaped to minimize spillover and radiates with a polarization vector that is parallel to the illuminated reflecting rods. Though providing significant improvement over the prior art with respect to scan rate and beam shape, the antenna disclosed by Cronson et al requires mechanical rotation of the feed and thus exhibits a limitation on the maximum scan rate achievable. Additionally, this antenna is limited with respect to the achievable sidelobe level in the azimuth plane. The sidelobe level may be improved with the utilization of a plurality of appropriately weighted radiating elements in the feed system. This, however, increases the inertia of the feed system and further limits the azimuthal scan rate.
The present invention is directed to an improved circularly scanning antenna with a scan rate capability significantly higher than those achieved by the antennas of the prior art and in which a plurality of radiating elements is utilized in the feed system to achieve low sidelobe levels over an entire 360 degree scan range.